In recent years, the number of patients suffering from nephropathy tends to increase. The reasons therefor include change in living environment, aging and increase in the number of patients suffering from diabetic nephropathy accompanied by the increase in the number of patients suffering from diabetes mellitus. The number of patients whose renal function decreased to reach renal failure so that dialysis is inevitable is increasing year by year. Dialysis treatment requires the patient to attend a hospital twice or thrice a week. In addition, dialysis treatment has a number of problems including disorder in production and maturation of erythrocytes; emergence of complications due to accumulation of aluminum or β2-microglobulin accompanied by long-term dialysis treatment; and increase in pathologic change in cardiovascular system. Especially, in cases where the primary disease is diabetes mellitus, the survival rate after 5 years from the commencement of the dialysis treatment is as low as about 50%. Thus, a drug which suppresses the progress of renal diseases so as to extend the duration till the dialysis is strongly demanded.
To suppress progress of renal diseases, antihypertensive therapies are generally performed, in addition to diet therapies such as low protein diets. Moreover, in case of glomerulonephritis, steroid drugs or immunosuppressants are administered to suppress the inflammatory reaction, and in case of diabetic nephritis, insulin or oral antidiabetic agents are administered to attain strict control of blood glucose.
For patients who reached renal failure, drugs for suppressing increase in blood electrolytes are administered, and low protein diets are prescribed, in addition to the antihypertensive therapies. Further, in cases where renal anemia is complicated, erythropoietin is administered. Still further, to slow down the progression or to improve the uremia, oral adsorbent preparations may be used in some cases.
However, in spite of these therapies, the progression of renal failure cannot be well prevented at present.
Since renal diseases such as nephritis, diabetic nephropathy and renal failure often accompany hypertension, and since hypertension is thought to be one of the factors which aggravate the renal diseases, antihypertensive drugs are administered for the purpose of suppressing progress of the renal diseases. Among the antihypertensive drugs, inhibitors of the renin-angiotensin system are especially drawing attention. Angiotensin II having a high vasopressor activity is generated from angiotensin I by angiotensin converting enzyme (ACE). Therefore, substances which inhibit ACE have antihypertensive activities and are widely used as antihypertensive drugs. However, it is known that ACE inhibitors commonly have side effects such as dry cough. Another type of inhibitors of the renin-angiotensin system include angiotensin II receptor antagonists. The receptor of angiotensin II is known to include two subtypes, AT1 and AT2. Antagonists of AT1 receptor have been widely used as antihypertensive drugs having fewer side effects than ACE inhibitors.
It has been reported that ACE inhibitors and angiotensin II receptor antagonists suppress the progress of chronic glomerular nephritis and diabetic nephropathy in animal models and in human (Am J Med 1995 November; 99(5): 497-504, Diabetologia 1996 May; 39(5): 587-93, N Engl J Med 2001 Sep. 20; 345(12): 861-9, J Hypertens 1993 September; 11(9): 969-75). Further, it is thought that ACE inhibitors and angiotensin II receptor antagonists also have renal protective activities which are not related to the antihypertensive activities. For example, the effect of suppressing the progress of diabetic nephropathy by irbesartan was superior to that attained in cases where the blood pressure was controlled by a calcium antagonist (N Engl J Med 2001 Sep. 20; 345(12): 851-60). Therefore, particularly for the patients suffering from diabetic nephropathy, ACE inhibitors and angiotensin II receptor antagonists are widely used even if the blood pressure is within the normal range.
Thus, the usefulness of the inhibitors of the renin-angiotensin system such as ACE inhibitors and angiotensin II receptor antagonists against renal diseases are clinically well recognized.
However, it has been shown that the effects of suppressing progress of renal diseases by ACE inhibitors and angiotensin II receptor antagonists are limited. For example, in a clinical test of losartan which is a representative angiotensin II receptor antagonist, for patients suffering from diabetic nephropathy, it was proved that the doubling time of creatinine, the rate of cases where dialysis became necessary, and complicated risk of death were decreased. However, the risk-lowering rate was only 16.1% (N Engl J Med 2001 Sep. 20; 345(12): 861-9). The fact that the degree of suppression is not sufficient is evident from the fact that the number of patients who are newly required to receive dialysis exceeds 30,000 in Japan and is ever-increasing year by year, in spite of the fact that ACE inhibitors and angiotensin II receptor antagonists are widely used as antihypertensive drugs.
It has been reported that iloprost, a prostaglandin I derivative, decreased urinary protein in an anti-Thyl-induced nephritis model, which is a glomerular nephritis model (Am J Pathol 1993 February; 142(2): 441-50). Further, beraprost sodium, a prostaglandin I derivative, decreases urinary protein in glomerular nephritis model in rats (Kidney Int 1998 May; 53(5) 1314-20) and in patients suffering from diabetic nephropathy (Nephron 2002 December; 92(4): 788-96). Still further, it has been reported that cicaprost suppressed the diabetic nephritis induced by streptozotocin in rats (J Hypertens Suppl 1993 December; 11 Suppl 5: S208-9) and the renal dysfunction induced by uninephrectomy and by loading high sodium and high protein (Am J Hypertens 1997 10: 209-16).
WO 00/67748 discloses that m-phenylene PGI2 derivatives including beraprost sodium are effective for the therapy of renal failure. WO 99/13880 discloses that m-phenylene PGI2 derivatives including beraprost sodium are effective for nephritis, glomerular nephritis and diabetic nephropathy. WO 02/080929 discloses that m-phenylene PGI2 derivatives including beraprost sodium are effective for interstitial nephritis.
However, those publications merely disclose the effect of the prostaglandin I derivatives against the progress of nephropathy when administered individually, and the references are totally silent about the combination of prostaglandin I derivative and an inhibitor of the renin-angiotensin system.
A novel composition containing inter-phenylene-9-thia-11-oxo-12-azaprostanoic acid which is a novel compound of the prostanoic acid type, which has a special structure different from the native form, and an inhibitor of angiotensin converting enzyme. This reference discloses that these compounds have very strong vasodilating activities for renal blood vessels. However, the compound disclosed in this patent literature is different from prostaglandins, and the patent literature is totally silent about whether the compound enhances the suppressive effect of the inhibitors of the renin-angiotensin system against renal diseases (Japanese Laid-open Patent Application (Kokai) No. 60-23324).
Further, cicaprost, a prostaglandin I derivative, and fosinopril, an ACE inhibitor, were administered in combination to diabetes mellitus model in rats, and the progression of the diabetic nephropathy was evaluated (Am J Hypertens 1997 10: 202-208). In this literature, it is described that when each drug was administered individually, the renal function parameters such as urinary protein were less severe and the damage of renal tissue was lighter than the control group, but in cases where both of the drugs were administered in combination, the effect was not more than that attained in cases where each drug was administered individually, so that no synergistic effect was observed.
It could therefore be helpful is to provide an agent for enhancing therapeutic or prophylactic effect of administering renin-angiotensin system inhibitor on renal diseases.